Understanding the mechanisms for the breakdown of self-tolerance in autoimmunity is essential for the development of strategies to prevent and treat autoimmune diseases. Abundant evidence has shown that defective apoptosis in the immune system is associated with the development of systemic autoimmune diseases in humans and mice. However, inhibition of apoptosis in lymphocytes alone is not sufficient to break immune tolerance, indicating that impaired apoptosis in other cell types plays a critical role in the breakdown of self-tolerance. Targeted inhibition of apoptosis in dendritic cells (DCs) has been shown to induce systemic autoimmune responses. Experiments are proposed to test the hypothesis that apoptosis in DCs is essential for limiting lymphocyte activation and preventing autoimmunity in the following specific aims: 1) to characterize the apoptosis pathways in DCs. Death receptor-mediated and mitochondrion-dependent apoptosis pathways will be characterized in DCs. Preliminary studies suggested that the lifespan of DC subsets in vivo was correlating to the molecular ratios between anti-apoptotic and pro-apoptotic bcl-2 family members. Experiments will be performed to further characterize the bcl-2-regulated mitochondrial apoptosis pathways in regulating DC apoptosis;2) to test the hypothesis that defective apoptosis in DCs contributes to dysregulated lymphocyte activation. The lifespan of DCs can potentially influence immune responses by affecting the duration of DCs in stimulating lymphocytes. The potentials for apoptosis-deficient DCs in over- activating lymphocytes will be examined;and 3) to test the hypothesis that defective apoptosis in DCs contributes to the development of autoimmunity. Studies will be performed to examine whether DCs harboring apoptosis deficiency in the mitochondrial apoptosis pathways leads to the development of autoimmunity. Experiments are proposed to test the hypothesis that apoptosis regulates DC homeostasis and immunogenicity, and defective apoptosis in DCs contributes to the onset of autoimmunity. In the long term, the knowledge gained from these studies will be used to develop more specific and effective strategies to prevent the onset of autoimmunity by targeting apoptosis in DCs.